Long-term effects of Fe3O4 NPs on the granule-based anaerobic ammonium oxidation process: Performance, sludge characteristics and microbial community

The performance of anaerobic ammonium oxidation (anammox) granules were studied under long-term exposure to Fe3O4 NPs. The Fe3O4 NPs had no negative impacts on nitrogen removal performance with the addition of 2-200 mg L-1. The specific anammox activity (SAA) slightly decreased from 287.0 +/- 13.2 to -253.0 +/- 9.2 mg TN g(-1)VSS d(-1) with the increase in Fe3O4 NPs level from 2 to 60 mg L-1, and then significantly enhanced to 381.8 +/- 15.7 mg TN g(-1)VSS d(-1) at 200 mg L-1 Fe3O4 NPs. And the change trends of the heme c content, extracellular polymeric substance amount and settling velocity were consistent with that of SAA. The Candidatus_Kuenenia was the dominant species during the entire experiment and its relative abundance was up to 33.4 % at the end the experiment. The results provide some useful information for comprehending the impact of Fe3O4 NPs on the performance of wastewater biological treatment systems.